This invention relates generally to terminator devices and more particularly, it relates to an improved terminator method and apparatus used with a SCSI (acronym for small computer system interface) bus line for removing noise spikes or transients from data and/or control signals received on the bus line.
One application where reliable data transfer becomes important is in the field of data transmission on a fully loaded SCSI bus line for communication between a plurality of data drives and a central processing unit (CPU) in a small computer. The basic problem encountered when transferring data on the SCSI bus line is due to spurious or random narrow pulses referred generally to as transients or noise spikes appearing in a stream of data signals or on control signals. Since the data or control signal is essentially a plurality of pulses having either a high or low value, these transients may result in erroneous data acquisition, undesired triggering, or false signals to be generated.
The transients are more hazardous when they appear on the REQ and ACK signal lines. The control signals REQ and ACK are used to perform a "handshake" so as to transfer the data back and forth between a target (i.e., a disk drive) and an initiator (i.e., a host computer). For instance, a transient occurring in the control signal REQ may cause double strobing of the data. Also, the incorrect or wrong data may be transferred when an early control signal REQ occurs and the data signal lines have not yet completely settled.
As is generally known in the art of computer equipment manufacturing, a termination device is typically connected to each end of an OR-WIRED SCSI bus line for supplying a fixed supply voltage with a predetermined impedance. In FIG. 1, there is shown prior art termination networks 10 and 12 connected to the respective ends of the OR-WIRED SCSI bus line 14. Each of the termination networks 10, 12 includes a voltage divider formed of two resistors T1 and T2 connected in series. One end of the resistor T1 is connected to a power supply voltage TERMPWR, which is typically at +5.0 volts. One end of the resistor T2 is connected to a ground potential GND, which is typically at zero volts. The junction of the resistors T1 and T2 for the termination network 10 is connected to one end of the bus line 14, and the junction of the resistors T1 and T2 for the termination network 12 is connected to the other end of the bus line. For the SCSI bus line, the values of the resistors T1 are 220.OMEGA. and the values of the resistor T2 are 330.OMEGA. so as to provide approximately +3.0 volts at the junction points J1 and J2 when the bus line is not active.
The OR-WIRED SCSI bus line 14 has a plurality of transceivers 16a, 16b, and 16c which are connected thereto by respective signal lines 18a, 18b, and 18c. Each of the transceivers 16a-16c includes a transmitting device 20 having an open collector output (i.e., NAND logic gate type 7438) and a receiving device 22 (i.e., Schmitt trigger input type 7414). When the bus line is not active, the signal line (i.e., line 18a) is at a high or logic "1" level which has a voltage value of approximately +3.0 volts. A low or logic "0" level is obtained when an open collector output device is turned on so as to pull the voltage value on the signal line down to approximately zero volts (i.e., 0.2 volts through the driver transistor Q1).
While there are many different arrangements of the termination network that can be used to achieve the above results, there is a requirement of ANSI specification (X3T9.2/86-109REV10c) which states that any one termination network shall provide no more than a current of 24 mA during the logic "0" level. One such alternative arrangement is the prior art termination network 10a which is shown in FIG. 2. This alternative arrangement is referred to as a "regulated terminator" which includes a voltage regulator 24 for receiving the voltage TERMPWR on line Vin and for generating a regulated voltage on line Vout of approximately +2.85 volts with 110 ohms resistors to the respective 18 signal lines DB(0)-DB(7), ATN, BSY, ACK, RST, MSG, SEL, C/D, REQ and I/O. With the 110 ohms-resistors P1-P18 functioning as pull-up resistors, the maximum current requirement of the open collector driver transistor is met.
The principal unsolved problem of this prior art regulated terminator 10a was that it was still susceptible to noise spikes or transients appearing on the SCSI bus line. Further, it has been encountered in the computer equipment industry that there are more than 100 varieties of SCSI cables having impedances ranging between 45 ohms to 132 ohms. Thus, this prior art regulated terminator did not suggest or hint as to how it could accommodate the different types of SCSI cables.
It is also known in the prior art that a termination technique employing Schottky diodes may be used for reducing of line ringing. Such techniques are illustrated on pages 77-81 of a manual entitled "MECL System Design Handbook" published by Motorola, Inc. of Schaumburg, Ill. A typical diode termination circuit obtained from this manual is illustrated in FIG. 3. However, it should be apparent that such diode termination circuit is not even remotely concerned with the problems associated with a termination network, i.e., providing a fixed voltage with a termination resistor so as to meet the maximum current requirements previously discussed.
Accordingly, there has arisen a need in the computer equipment industry for an improved terminator apparatus for removing noise spikes or transients occurring in the control signals REQ and ACK on the SCSI bus line so as to provide reliable and accurate data transmission. It would also be expedient that the terminator apparatus be capable of being readily modified so as to accommodate a range of SCSI cable impedances.